why is a step up transformer used

I think you might be confusing a step up transformer for A/C power with a SUT for phono?

An A/C step up lets you use a lower voltage device, like a 100 volt Japanese unit, in the US or other country.

A SUT, takes the very low voltage of a cartridge and boosts it, so that a normal phono stage can use it.

The Krell KPE has internal adjustable gain settings. It should be able to drive almost any cartridge out there.

Here is the manual:

http://www.krellonline.com/assets/support/0057_017_MAN.pdf

Sorry, I wasn't as clear as I wanted to be in my comments above.

The Krell KPE phono stage could only be powered from the Krell preamp's power supply or an external power supply. The external power supply didn't give the KPE any extra gain, (for the cartridge) than the power supply from the preamp. It just saved the owner some money if they already had the preamp.